1. Field of the Invention
The present invention relates to an acceleration detection device for use in an acceleration detection sensor.
2. Description of the Related Art
FIG. 11 shows an example of a conventional acceleration detection device 18. The acceleration detection device 18 comprises a board 20, a support section 25, a beam 21 in the shape of a cantilever, a weight 22, movable electrodes 23a and 23b, and fixed electrodes 24a and 24b. Referring to FIG. 11, the support section 25 is securely formed on the board surface of the board 20, with the base end of the beam 21 being connected to the support section 25. The beam 21 is formed so as to extend horizontally along the board surface with a gap between it and the board surface of the board 20, with the weight 22 being provided at the extension end of the beam 21. The weight 22 is formed symmetrically between the upper and lower halves with respect to the center axis along the length of the beam 21, with the position of the center of gravity G of the weight 22 at the center axis. The movable electrode 23a is formed on the surface of weight 22 facing the board, and the movable electrode 23b is formed on the opposite surface of weight 22. Further, the fixed electrodes 24a and 24b are provided which face the movable electrodes 23a and 23b, respectively, each with a gap therebetween.
Connected to the movable electrodes 23a and 23b and the fixed electrodes 24a and 24b are detection means (not shown) for performing detection by applying a voltage between the movable electrodes 23a and 23b and the fixed electrodes 24a and 24b which face each other and by converting the electrostatic capacity between the movable electrodes 23a and 23b and the fixed electrodes 24a and 24b into a voltage.
The acceleration detection device 18 constructed as described above is designed to detect an acceleration in a direction (in the Y direction in the example shown in the figure) perpendicular to the board surface of the board 20. When an upward and downward acceleration (an upward and downward acceleration in the Y direction) in the direction shown in FIG. 11, perpendicular to the surface of the board 20, is applied, an upward and downward inertial force in the Y direction corresponding to the direction and the magnitude of the acceleration occurs (to be specific, when an acceleration occurs upwardly, the inertial force occurs downwardly; when, on the contrary, an acceleration occurs downwardly, the inertial force occurs upwardly). This inertial force causes the beam 21 to be flexibly deformed, causing the weight 22 to be displaced. That is, the movable electrodes 23a and 23b are displaced integrally with the weight 22, and the electrode-to-electrode distance between the movable electrodes 23a and 23b and the fixed electrodes 24a and 24b varies. For this reason, the electrostatic capacity between the movable electrodes 23a and 23b and the fixed electrodes 24a and 24b varies. The magnitude of the upward and downward acceleration or the like is detected on the basis of the amount of such change of the electrostatic capacity.
However, in the acceleration detection device 18 having the above-described construction, since the position of the gravity G of the weight 22 is on the center axis of the beam 21, when an acceleration along the length (in the X direction) of the beam 21 is applied, an inertial moment does not act on the beam 21, and the weight 22 is not displaced. Therefore, the electrostatic capacity between the movable electrodes 23a and 23b and the fixed electrodes 24a and 24b does not vary, and therefore, an acceleration in the X direction cannot be detected. That is, in the acceleration detection device 18 constructed as described above, only the acceleration in the Y direction perpendicular to the surface of the board 20 can be detected. Of course, it is not possible to detect the magnitude of acceleration in two or more directions.